Aluminum-magnesium alloy sheet



United States Patent O 3,359,085 ALUMINUM-MAGNESIUM ALLOY SHEET William A. Anderson, Verona, Pa., assignor to Aluminum Oompany of America, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Filed June 2, 1964, Ser. No. 372,095

9 Claims. (Cl. 29-1975) This invention relates to the production of work hardened aluminum alloy sheet which possesses consistently good strength and workability after partial relaxation of the work hardening strains. More particularly, the invention relates to the production of an improved work hardened aluminum-magnesium alloy sheet that, further, exhibits a gradual, substantially uniform, change in yield strength (temperature vs. strength-ductility) between the fully work hardened and fully annealed conditions when subjected to thermal strain relieving treatments and to a process for so treating the sheet.

Aluminum-magnesium alloys are well known for their high strength and good corrosion resistance properties and have thus found wide commercial acceptance, especially those containing less than 6.5% magnesium. The latter alloys are generally not solution heat treated because little or no improvement in strength is realized thereby, but they can be very appreciably strengthened by cold rolling or other cold working operations as compared to the strength in the annealed condition.

It is known that cold worked aluminum-magnesium type alloys can be softened by suitable thermal treatment and that the extent of the softening depends upon the temperature and length of exposure to the temperature. However in commercial practice, the temperature is often increased to the threshold of recrystallization where an extremely rapid and disproportionate decrease in strength occurs. Obviously, no predictable strength-ductility levels result from temperature exposures at or near this region of rapid change.

The severity of the problem is related to the degree of strain hardening imparted to the sheet. The greater the degree of strain hardening, the more abrupt is the decrease in strength in the region where recrystallization occurs. Further, this change will occur at lower and less repeatable temperatures as the severity of work hardening increases. Prior aluminum-magnesium alloys containing chromium and/or manganese when cold rolled with a reduction of 85 or 90% or more, are especially sensitive in this respect to temperature exposures over 300 F. The problem becomes even more acute as cold reduction facilities, such as continuous high speed rolling mills are developed and improved. Such will impose even more severe demands on alloy sheet in the reduction operations and result in even greater strain hardened tempers.

As is almost invariably the case in a dynamic industrial pattern, the alloy sheet, to offer the desired utility,

will frequently be heated to temperatures beyond those which are considered to provide predictable or repeatable resultant strength values. An example of such an application occurs in food container can end fabrication fro-m aluminum-magnesium alloy sheet. Aluminum-magnesium alloy sheet is recognized as offering many advantages in this application because of its high strength and corrosion resistance, and its feasible cost. In many cases the sheet is heated to a temperature level above the threshold of recrystallization, for example, above 400 R, where, with sheet of relatively severe strain hardened temper, the resultant strength values are not predictable and thus cause serious problems in the various subsequent working operations.

Accordingly, it is a principal object of the invention to provide for an aluminum-magnesium alloy sheet of extra hard temper possessing relatively high strength and good workability after exposure to temperatures below the annealing temperature and, further, capable of responding to strain relief heat treatments thereby producing reliable and repeatable strength values at any level between full hard and dead soft tempers or, conversely, exhibiting predictable strength values at given temperatures.

Another object of the invention is to furnish sheet pos sessing these qualities, the sheet being economical and suitable for commercial applications.

A further object of the invention is to provide a process for strain relieving aluminum-magnesium alloy sheet in the severe strain hardened temper to any desired strengthductility level with reproducible results.

A still further object is to provide for aluminummagnesium alloy sheet of extra strain hardened temper, containing from 4% to 5%, by weight of magnesium, the sheet exhibiting relatively high strength and good work ability after exposure to temperatures below the annealing temperature, and further, exhibiting predictable and repeatable strength levels after a given thermal treatment.

Basically, the invention resides in providing a relatively thin sheet of aluminum-magnesium alloy containing from 4% to 5% magnesium, the sheet having a thickness ranging from 0.004 inch to 0.032 inch and in a relatively severely work hardened condition produced by a reduction in thickness by cold rolling of at least and preferably or more. The alloy, as mentioned above, should contain from 4% to 5% by weight of magnesium, preferably 4.2% to 4.8%, balance aluminum and impurities, the latter being below the following upper limits: 0.08% manganese, 0.03% chromium, 0.3% silicon, 0.5% iron, 0.25% copper, 0.5 zinc, 0.1% titanium, 0.2% zirconium, others, each 0.05% total of others not over 0.15%. Of extremely critical importance are the aforesaid maximum limits on manganese and chromium. In a preferred embodiment of the invention, the manganese content is further restricted to a maximum of 0.05%. While the above upper limits on manganese and chromium content ofler maximum improvement, substantial advantages will still be realized by limiting chromium and manganese, respectively, to less than 0.05% and 0.10%, without departing from the scope of the invention. The combined total amount of all impurities including manganese and chromium but excluding copper and zinc should not exceed 1% by weight of the alloy composition. Further, it is preferable that at least 0.1% of the impurities be present, as such improves the strength of the alloy at hot working temperatures thus tending to eliminate break-up or shear failures in sheet or plate during hot working operations. Obviously, this poses no problems as some impurities are almost always present in commercial practice. If the impurity content exceeds that set forth above, the alloy sheet will loose the desired benefits and to a significant extent revert to the performance level of prior alloy sheet in commercial applications. The limits on magnesium are based on strength and ductility or workability. If the magnesium content is substantially less than about 4% by weight, the strength level achievable by strain hardening is diminished to the extent that the commercial usefulness of the alloy sheet in relatively severe applications is seriously limited. As the magnesium content goes above 5%, the alloys ductility is excessively impaired for commercial purposes.

As indicated above, the limits on chromium and manganese impurities are of critical importance. It is believed that this is attributable to the occurrence of these elements in the form of a dispersoid phase, much like that of sand in concrete. The size of the dispersoid particles range from about 50-100 A., and they remain substantially out of solid solution. As is known, when the aluminum-magnesium alloy sheet is in the severe cold worked temper condition, its grains exhibit considerable fragmentation.

Upon gradual heating to the fully annealed condition where the grains are of relative uniform size and considerably larger, it would be desirable, from a standpoint of predictable and repeatable intermediate strength values, to have the grains or grain boundaries grow at a relatively uniform rate. However in the prior hard rolled tempers of aluminum-magnesium alloy sheet, such does not occur which is attributed to the dispersoid phase panticles of manganese and chromium functioning as anchor points which retard and terminate the growth of subgrain boundaries. This retardation of grain growth may be described as simply postponing the inevitable collapse and reversion of the subgrain system to the annealed grain structure. Thus there exists an unstable condition where the grain growth lags behind the temperature thereby storing strain energy. At recrystallization temperatures, the grain growth is accelerated as the anchor effect of the dispersoid phase is finally overcome and the built-up residual strain is suddenly relieved. This causes the sudden strength decrease associated with reaching these temperature levels. Critically limiting the amount of chromium and manganese, anchor points eliminates or minimizes this condition at least to the extent that a relatively steady rate of strength decrease is exhibited with increasing temperature exposures. This theoretical explanation is offered to illustrate the invention with greater perspective but is not intended, in any manner, to place a limitation on the scope of the invention. The invention and its advantages are observable facts.

In order to form aluminum alloy sheet in accordance with the invention, an ingot of the alloy composition is hot rolled to the desired thickness, according to the usual practice, then annealed and cold rolled with a reduction in thickness of at least 85% to yield sheet from 0.004 to 0.032 inch in thickness.

This cold rolled sheet will exhibit a tensile strength ranging from about 49,000 to 65,000 p.s.i. and a yield strength of 47,000 to 63,000 or higher p.s.i. depending on the extent of the cold reduction and composition of the alloy. 'For example, alloy sheet of a typical composition, 4.5% magnesium, 0.05% manganese, 0.03% chrornium, 0.16% iron, 0.06% silicon, 0.01% zinc, and ba1- ance aluminum, exhibits a tensile strength of over 60,000 p.s.i. and a yield strength of about 59,000 p.s.i. after a reduction of 91% by cold rolling. Even in this severe cold worked condition, the sheet generally has adequate workability, since it is characterized by a 2% minimum elongation, thus rendering it highly useful for fabrication of many useful shapes without further heat treatment.

Although the foregoing cold rolled sheet can be used for many purpose-s, it is often desirable to employ sheet that has a greater ductility even at the sacrifice of some strength. To achieve this condition, the alloy sheet is heated to from 200 F. up to a level where very little, if any, recrystallization has occurred, about 475 F.

The effect of heating 0.014 inch thick sheet of the typical composition to different temperatures for a period of two hours is illustrated in the results given in Table I below. The sheet was, of course, cooled to room temperature before the tensile tests were made.

TABLE I..TENSILE PROPERTIES OF IKE-HEATED SHEET As is apparent in Table I, a wide range of strength and elongation values were achieved by the various treatments. In all cases but the last, the heating relieved some of the work hardening strains but with little, if any, recrystallization. The relationship of yield strength to temperature of treatment is substantially linear.

The effect of a given length of time and temperature upon yield strength may be determined empirically for the specific conditions involved. This effect will be consistently obtained, even in large scale operations, so long as the given conditions are followed. Deviations caused by lot changes or minor composition variations are generally inconsequential. For example, if a 15 minute exposure at 300 F. is determined by experiment to result in a yield strength of about 51,000 p.s.i. and a 5% elongation, then this effect can be repeated time after time in mass production operations without sizeable deviations.

In room temperature yield strength response to exposure, temperature, a significant contrast is offered by comparing sheets of the typical composition set forth above for Table I to a like composition, but containing excessive manganese and chromium in amounts of, respectively, 0.14% and 0.08%, both sheets having been cold rolled with a thickness reduction. The sheet containing excessive chromium and manganese will exhibit steadily diminishing resultant yield strength with exposures to increasing temperatures in the lower temperature range. However, as the temperature is increased to around 400 F., an abrupt and disproportionate strength decrease occurs wherein strength very rapidly diminishes. For example, over a 35 F. temperature range, the resultant yield strength may be decreased from 39,000 p.s.i. to 22,000 p.s.i. Further, this abrupt change occurs at unpredictable and unrepeatable temperatures most often ranging from 350 F. to 475 F. Considering two samples of the same alloy sheet, one may be in the region of the change at 390 F., the other not reaching this region till 450 F. On the other hand, my alloy sheet will exhibit resultant yield strengths steadily diminishing with increasing exposure temperatures. This substantially linear and uniform rate, typified by the data set forth in Table I, exists for exposure temperatures varying from 200 F. to 475 F. and is particularly significant over a temperature range of 350 F. to 475 F. This uniform reduction in resultant yield strength with increasing exposure temperature is highly continuous and steady and also, it is emphasized, provides repeatable and reproducible results. This reproducibility is particularly important in commercial fabricating operations involving large quantities of sheet.

The upper temperature limit of 475 F. is used since, at higher temperatures, little commercial utility is realized from the treatment. If this temperature is substantially exceeded, the resultant strength is not significantly higher than the annealed state. For example, a 2-hour exposure at 475 F. will result in the sheet having a yield strength of about 29,000 p.s.i. or more and an elongation of 15% or more thus retaining a substantial portion of the strain hardening imparted by the cold rolling, whereas at 500 F. the yield strength drops to 23,000 p.s.i. A temperature of 200 F. is the lower temperature limit since at values substantially lower, there is but little change in yield strength from that possessed by the cold rolled sheet prior to the thermal treatment. The temperatures referred to are those attained in the sheet itself as distinguished from the furnace temperature.

The length of the period at which the sheet is held at the desired temperature will vary with the size of the load, the nature of the furnace and other well-known factors. For commercial applications, the time and temperautre parameters are manipulated in the customary manner, bearing in mind the general guidelines set forth below. Generally a thin sheet will realize almost all the relaxation of the work hardening strain for a given temperature after about two to five minutes at that temperature. For temperatures below 400 F., extended hold times generally result in relatively insignificant further relaxation. As the temperature is further increased, time becomes more important since, now at the threshold of recrystallization, grain growth progresses with both time and temperature. Thus, depending on the thickness of the sheet, temperatures over 400 F. may be held up to 4 hours but preferably not over 2 hours. Beyond this point, resulting strength levels may vary because of varying degrees of recrystallization. In a modern commercial application, it may be desired to pass a 0.014 inch thick sheet through a 700 F. zone for about one minute where the sheet achieves about a 450 F. temperature for most of that period. Within this period, the sheet will realize most of the relaxation of the hardening strain associated with a 450 F. temperature exposure. Thus intermediate tempers, between the initial strain hardened temper and a second, lower strength, temper are produced by the practice of the invention. For the preferred composition applicable to Table I, this second temper, that which results from a 475 F. exposure, is characterized by a yield strength of 29,500 p.s.i. which is about 50% of the initial strength of 58,900 p.s.i. Obviously the yield strength of the second temper will depend on the strength of the initial temper and is set forth generally, as being over 45% of the initial strength. This second temper may be contrasted to the annealed state which, for example, in Table I it exhibits a yield strength of about 20,000 p.s.i., about 34% of the initial strength. In other words, intermediate strength levels between the initial level and 45% thereof may be produced by temperature exposures of 200 F. to 475 F. by the practice of the invention. Conversely, within these ranges, the resultant yield strength to be expected for a given temperature exposure may be accurately predicted.

In order to better understand the nature and advantages of the alloy sheet and its many uses, an application embodied in a can end is described. Aluminum-magnesium sheet containing 4% to 5% magnesium is highly useful as can stock for use in canning beverages and other foodstuff, either in conventionally designed cans or in cans featuring a tear open feature. An example is an aluminum can end for use with a can body of tin plated steel or aluminum. In forming the can end the alloy sheet is rolled to a reduction of 90% or more resulting in a thickness of about from 0.010 inch to 0.016 inch or nominally about 0.014 inch and exhibits the extra hard temper. The sheet would be subjected to a resin or lacquer coating, a drying, a stamping and various shaping operations. The sheet is sprayed with a coating, generally lacquer, to improve its corrosion resistance on its inside can surface, then passed through a lacquer drying operation where it is heated to temperatures approaching 400 F. Because of these temperatures and because of the flat and unrestrained nature of the sheet, the heat may result in distortion of the sheet as hardening strains are relaxed. Therefore it is often advantageous to preheat the sheet to prevent this distortion, which otherwise often hinders subsequent handling and forming operations. Thus, prior to coating the sheet may be heated to a temperatur of about 425 F., a temperature above that expected in drying. Heating prior aluminum-magnesium alloy can stock sheet to such temperature levels results in extremely unpredictable strength levels. However, my alloy sheet exhibits entirely consistent and predictable strength characteristics thus rendering subsequent forming operations reliable and consistent.

Proceeding with the example pertinent to the can stock, the dried coated sheet is next stamped into disc-like shapes about 2 /2 or 3 inches in diameter. Following this stamping, various forming operations are imposed upon the sheet to achieve the desired final configuration. These operations might include stamping, pressing, scoring, or even drawing a rivet from the sheet disc in order to secure a tear open tab or handle. This relatively severe operation may be performed on the thermally treated alloy sheet generally with relative ease attributable to its superior workability. Further, these operations will yield consistent results attendant to the consistency of the strength and ductility qualities of the alloy sheet after temperature exposures.

If desired, the alloy sheet may be produced in the form of a composite comprising the alloy sheet as a core and a cladding on one or both surfaces. The cladding would account for from. 2 /2% to 15% of the total thickness for each surface clad and would be metallurgically bonded, as by hot roll bonding, to the core prior to the cold rolling which reduces the thickness of the core by at least The cladding is of an aluminous metal, i.e. aluminum or a suitable alloy thereof. Usually it is de sirable to select a cladding metal which is anodic to the core thus providing it with galvanic corrosion protection.

Example 1 In the manner set forth hereinbefore, an aluminummagnesium alloy ingot of the typical composition set forth above for Table I is hot rolled to strip having a thickness of 0.120 inch. The hot rolled strip was fully annealed at 700 F. and then cold rolled by a continuous high speed rolling operation to a thickness of about 0.011 inch, which represented a reduction of about 91%. The alloy sheet thus produced had a tensile strength of over 60,000 p.s.i. and a yield strength of about 59,000 p.s.i. in this extra strain hardened condition and had an elongation of almost 2 /2%. It also had superior workability which is evidenced by the fact that the sheet could be cold rolled to a 91% reduction without rolling defects. Further, this sheet exhibits a highly continuous and steady relation between exposure temperatures and resultant yield strength values thus permitting its strength to be decreased to any level between its relatively high original work hardened level and a second, somewhat lower strength level, exhibiting about 29,500 p.s.i. yield. This continuity as illustrated by the data of Table I prevails even at temperatures over 400 P. where prior aluminummagnesium alloy sheet of even lower cold worked tempers exhibits such sudden and uncontrollable strength losses.

Example 2 In the manner set forth in Example 1, an aluminummagnesium alloy sheet was prepared having a thickness of about 0.014 inch, the only difference in composition being that the chromium content was 0.023 percent and the manganese content was about 0.075 percent. This alloy sheet exhibited similar strength and substantially the same degree of steadiness and continuity between exposure temperature and diminishing yield strength as the sheet described in Example 1 and has exhibited proven superiority over prior aluminum-magnesium alloy sheet in can stock applications. Thus this sheet, having a thickness of about from 0.010 inch to 0.016 inch or nominally about 0.014 inch is a preferred embodimentof the invention from this important commercial standpoint.

It can be seen from the above description and examples that the objects of the invention have been accomplished as set forth above. There has been shown a process of producing aluminum-magnesium alloy sheet in any desired temper and has described with particularity the sheet and process. This sheet will find use in highly diversified applications because of its high strength and its predictable and reproducible properties after temperature exposures.

It is to be understood that this description is illustrative of, and not in limitation of the invention.

What is claimed is:

1. A work hardened aluminum-magnesium alloy sheet of 0.004 inch to 0.032 inch in thickness that has received a reduction in thickness of not less than 85% by cold rolling from the fully recrystallized state, said aluminummagnesium alloy consisting of 4.2% to 4.8% magnesium, the balance being aluminum and a total of at least 0.1%

impurities, the maximum amounts of said impurities being as follows: 0.05% chromium, 0.10% manganese, 0.5% iron, 0.25% copper, 0.3% silicon, 0.5% zinc, 0.1% titanium, 0.2% zicronium, and all other impurities being limited to 0.05% each and 0.15% total, the combined total of all the impurities, excluding zinc and copper, not exceeding 1%, said sheet being characterized by a yield strength of at least 47,000 p.s.i. in the original work hardened level and exhibiting a steady relation between exposure temperature and resultant yield strength such that when the said sheet is exposed to temperatures of from 200 F. to 475 F. its strength is decreased to a level between the original work hardened level and a second level characterized by a yield strength of at least 45% of that. of the initial level, the said resultant yield strength being substantially directly related to the exposure temperature in a highly continuous, steady and repeatable manner.

2. A work hardened clad sheet having at least one surface of an aluminous metal cladding which comprises from 2 /2% to 15% of the total clad sheet thickness for each surface clad, said clad being metallurgically bonded to a core sheet, said core sheet being an aluminum-magnesium alloy sheet of 0.004 inch to 0.032 inch in thickness that has received a reduction in thickness of not less than 85% by cold rolling from the fully recrystallized state, said aluminum-magnesium alloy consisting of 4.2% to 4.8% magnesium, the balance being aluminum and a total of at least 0.1% impurities, the maximum amounts of said impurities being as follows: 0.05% chromium, 0.10% manganese, 0.05% iron, 0.25% copper, 0.3% silicon, 0.5 zinc, 0.1% titanium, 0.2% zirconium, and all other impurities being limited to 0.05% each and 0.15% total, the combined total of all the impurities, excluding zinc and copper, not exceeding 1% said core sheet being characterized by a yield strength of at least 47,000 p.s.i. in the original work hardened level and exhibiting a steady relation between exposure temperature and resultant yield strength such that when the said core sheet is exposed to temperatures of from 200 F. to 475 F. its strength is decreased to a level between the original work hardened level and a second level characterized by a yield strength of at least 45 of that of the initial level, the said resultant yield strength being substantially directly related to the exposure time and temperature in a highly continuous, steady and repeatable manner.

3. The method of producing an intermediate temper in an aluminum-magnesium alloy sheet 0.004 inch to 0.032 inch in thickness which has been cold rolled with a reduction in thickness of at least 85% and possesses a minimum yield strength of 47,000 p.s.i. in this original work hardened temper, said alloy consisting of 4% to 5% magnesium, the balance being aluminum and impurities, the maximum amounts of said impurities being as follows: 0.05% chromium, 0.1% manganese, 0.5% iron, 0.25% copper, 0.3% silicon, 0.5% zinc, 0.1% titanium, 0.2% zirconium, and all other impurities being limited to 0.05 each and a 0.15% total, the combined total of all the impurities, excluding zinc and copper, not to exceed 1%,

said method comprising exposing said alloy sheet to a temperature of from 200 F. to 475 F. for a length of time suflicient to effect a partial relaxation of the work hardening strains in the initial temper, such that the yield strength is substantially directly decreased to an intermediate value, between that of said initial temper and that of a second temper characterized by a minimum yield strength of at least 45% of that of the original temper.

4. The method of claim 3 wherein the magnesium content of the aluminum-magnesium sheet ranges from 4.2% to 4.8% by weight.

5. The method of claim 3 wherein the maximum limits on chromium and manganese in the aluminum-magnesium sheet are, respectively, 0.03% and 0.08%.

6. The method of claim 3 wherein the maximum limits on chromium and manganese in the aluminum-magnesium sheet are, respectively, 0.03% and 0.05%.

7. The method of claim 3 except that the thickness of the aluminum-magnesium sheet ranges from 0.010 inch to 0.016 inch.

8. The method of claim 3 wherein the aluminum-magnesium alloy sheet has been cold rolled with a reduction in thickness of at least 90%.

9. The method of producing aluminum-magnesium alloy sheet of intermediate temper comprising (1) cold rolling fully recrystallized stock with a reduction in thickness of at least the reduced thickness ranging from 0.004 inch to 0.032 inch, said alloy consisting of 4% to 5% magnesium, the balance being aluminum and impurities, the maximum amounts of said impurities being as follows: 0.05% chromium, 0.1% manganese, 0.5% iron, 0.25% copper, 0.3% silicon, 0.5% zinc, 0.1% titanium, 0.2% zirconium, and all other impurities being limited to 0.05% each and a 0.15% total, the combined total of all the impurities, excluding zinc and copper, not to exceed 1%, whereby an alloy sheet is produced in an initial temper having a minimum yield strength of 47,000 p.s.i. and (2) exposing said alloy sheet to a temperature of from 200 F. to 475 F. for a length of time sufiicient to effect a partial relaxation of the work hardening strains in the initial temper, such that the yield strength is substantially directly decreased to an intermediate value, between that of said initial temper and that of a second temper characterized by a minimum yield strength of at least 45% of that of the original temper.

References Cited UNITED STATES PATENTS 3,164,494 1/1965 English 148-315 2,841,512 7/1958 Cooper 148-11.5 3,187,428 6/1965 English 14811.5 X 3,235,961 2/1966 Champion et al. 29-197.5 X

FOREIGN PATENTS 431,054 6/1935 Great Britain.

CHARLES N. LOVELL, Primary Examiner.

DAVID L. RECK, Assistant Examiner. 

2. A WORK HARDENED CLAD SHEET HAVING AT LEAST ONE SURFACE OF AN ALUMINOUS METAL CLADDING WHICH COMPRISES FROM 2 1/2% TO 15% OF THE TOTAL CLAD SHEET THICKNESS FOR EACH SURFACE CLAD, SAID CLAD BEING METALLURGICALLY BONDED TO A CORE SHEET, SAID CORE SHEET BEING AN ALUMINUM-MAGNESIUM ALLOY SHEET OF 0.004 INCH TO 0.032 INCH IN THICKNESS THAT HAS RECEIVED A REDUCTION IN THICKNESS OF NOT LESS THAN 85% BY COLD ROLLING FROM THE FULLY RECRYSTALLIZED STATE, SAID ALUMINUM-MAGNESIUM ALLOY CONSISTING OF 4.2% TO 4.8% MAGNESIUM, THE BALANCE BEING ALUMINUM AND A TOTAL OF AT LEAST 0.1% IMPURITIES, THE MAXIMUM AMOUNTS OF SAID IMPURITIES BEING AS FOLLOWS: 0.05% CHROMIUM, 0.10% MANGANESE, 0.05% IRON, 0.25% COPPER, 0.3% SILICON, 0.5% ZINC, 0.1% TITANIUM, 0.2% ZIRCONIUMC AND ALL OTHER IMPURITIES BEING LIMITED TO 0.05% EACH AND 0.15% TOTAL, THE COMBINED TOTAL OF ALL THE IMPURITIES, EXCLUDING ZINC AND COPPER, NOT EXCEEDING 1% SAID CORE SHEET BEING CHARACTERIZED BY A YIELD STRENGTH OF AT LEAST 47,000 P.S.I. IN THE ORIGINAL WORK HARDENED LEVEL AND EXHIBITING A STEDY RELATION BETWEEN EXPOSURE TEMPERATURE AND RESULTANT YIELD STRENGTH SUCH THAT WHEN THE SAID CORE SHEET IS EXPOSED TO TEMPERATURES OF FROM 200*F. TO 475*F. ITS STRENGTH IS DECREASED TO A LEVEL BETWEEN THE ORIGINAL WORK HARDENED LEVEL AND A SECOND LEVEL CHARACTERIZED BY A YIELD STRENGTH OF AT LEAST 45% OF THAT OF THE INITIAL LEVEL, THE SAID RESULTANT YIELD STRENGTH BEING SUBSTANTIALY DIRECTLY RELATED TO THE EXPOSURE TIME AND TEMPERATURE IN A HIGHLY CONTINUOUS, STEADY AND REPEATABLE MANNER. 